Since c80e7c14e6305e50e3b39f97172d4d8fe1214d3b changed the GC heap to use
all unused RAM, there is no longer any RAM available for the traditional C
heap (which is not used by default in MicroPython but may be used by C
extensions). This commit adds a provision for a board to reserve RAM for
the C heap, by defining MICROPY_C_HEAP_SIZE.
Signed-off-by: Damien George <damien@micropython.org>